development and prospect of uav-based aerial electrostatic
TRANSCRIPT
applied sciences
Review
Development and Prospect of UAV-Based Aerial ElectrostaticSpray Technology in China
Yali Zhang 1,2 , Xinrong Huang 1,2, Yubin Lan 2,3, Linlin Wang 2,3, Xiaoyang Lu 1,2, Kangting Yan 1,2,Jizhong Deng 1,2,* and Wen Zeng 1,2,*
�����������������
Citation: Zhang, Y.; Huang, X.; Lan,
Y.; Wang, L.; Lu, X.; Yan, K.; Deng, J.;
Zeng, W. Development and Prospect
of UAV-Based Aerial Electrostatic
Spray Technology in China. Appl. Sci.
2021, 11, 4071. https://doi.org/
10.3390/app11094071
Academic Editor: Sylvain Bertrand
Received: 12 April 2021
Accepted: 28 April 2021
Published: 29 April 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1 College of Engineering, South China Agricultural University, Wushan Road, Guangzhou 510642, China;[email protected] (Y.Z.); [email protected] (X.H.); [email protected] (X.L.);[email protected] (K.Y.)
2 National Center for International Collaboration Research on Precision Agricultural Aviation PesticideSpraying Technology, Wushan Road, Guangzhou 510642, China; [email protected] (Y.L.);[email protected] (L.W.)
3 College of Electronic Engineering and College of Artificial Intelligence, South China Agricultural University,Wushan Road, Guangzhou 510642, China
* Correspondence: [email protected] (J.D.); [email protected] (W.Z.)
Abstract: Aerial electrostatic spray technology for agriculture is the integration of precision agri-cultural aviation and electrostatic spray technology. It is one of the research topics that have beenpaid close attention to by scholars in the field of agricultural aviation. This study summarizes thedevelopment of airborne electrostatic spray technology for agricultural use in China, includingthe early research and exploration of Chinese institutions and researchers in the aspects of nozzlestructure design optimization and theoretical simulation. The research progress of UAV-based aerialelectrostatic spray technology for agricultural use in China was expounded from the aspects of nozzlemodification, technical feasibility study, influencing mechanism of various factors, and field effi-ciency tests. According to the current development of agricultural UAVs and the characteristics of thefarmland environment in China, the UAV-based aerial electrostatic spray technology, which carriesthe airborne electrostatic spray system on the plant protection UAVs, has a wide potential in thefuture. At present, the application of UAV-based aerial electrostatic spray technology has yet to befurther improved due to several factors, such as the optimization of the test technology for chargeddroplets, the impact of UAV rotor wind field, comparison study on charging modes, and the lackof technical accumulation in the research of aerial electrostatic spray technology. With the contin-uous improvement of the research system of agricultural aviation electrostatic spray technology,UAV-based electrostatic spray technology will give play to the advantages in increasing the dropletsdeposition on the target and reducing environmental pollution from the application of pesticides.This study is capable of providing a reference for the development of the UAV-based agriculturalelectrostatic spray technology and the spray equipment.
Keywords: agricultural aviation; UAV; plant protection; review; electrostatic spray technology;droplet deposition; aerial pesticides application; charge to mess ratio
1. Introduction
Electrostatic spray technology for agricultural aviation is the application of traditionalelectrostatic spray technology in an airborne platform. It is one of the research topicsthat scholars in the field of agricultural aviation always pay attention to. Agriculturalaerial electrostatic spray technology is mainly based on induction, corona, and contactcharging methods to charge the droplets. Under the action of high voltage static electricity,charged droplets make rapid directional deposition along the electric field line in the airand settle on the target [1,2]. For this reason, aerial electrostatic spray can effectively reducedrift loss during aerial pesticide application, improve droplet deposition, and mitigate
Appl. Sci. 2021, 11, 4071. https://doi.org/10.3390/app11094071 https://www.mdpi.com/journal/applsci
Appl. Sci. 2021, 11, 4071 2 of 15
environmental pollution [3,4]. Carlton et al. [5] from the Agricultural Research Service ofthe United States Department of Agriculture (USDA-ARS) designed an electric rotatingelectrostatic nozzle in 1966, which was the first scientific research institution in the worldto carry out research on aerial electrostatic spray technology for agriculture. Carltonalso obtained the invention patent of the aerial electrostatic spray technology in 1999 [6].The patent was certified by the United States Department of Agriculture as the world’s firstand only commercially proven airborne electrostatic spray system for agriculture [7]. SES(Spectrum Stack Sprayers, Inc., Houston, TX, USA) has been awarded an exclusive licenseto manufacture and market this innovative technology. Subsequently, scientific researchinstitutions in the United States, Brazil, Canada, Switzerland, and China have conducted alarge number of studies to gradually improve the research and application of electrostaticspray in agricultural aviation [8–10].
The research of electrostatic spray technology for agricultural aviation started rela-tively late in China but developed rapidly. Especially in recent years, the rapid developmentof plant protection UAVs in China has attracted scholars to make a lot of attempts on the re-search and application of UAV-based electrostatic spray technology. This study introducedthe early exploration of aerial electrostatic spray technology in China from the aspects ofnozzle structure design optimization, theoretical simulation, and field experiment withmanned aircraft platforms. Then, the research progress of UAV-based electrostatic spraytechnology in China was emphatically expounded upon, and the existing problems werediscussed as well. It is proposed that future research should be carried out in the measure-ment technology for charged droplets, the influence of UAV rotor wind field on chargeddroplets, comparative study of various charging methods, and other aspects, so as toincrease the accumulation of research on aerial electrostatic spray technology based on anagricultural UAV platform. This study can provide a reference for the development of theagricultural aviation electrostatic spray technology and the spray equipment.
2. Early Exploration of Aerial Electrostatic Spray Technology in China
In the 1970s, China began to study the new technology of electrostatic spraying foragricultural application [11]. In 1977, Shenyang Spray Factory used hand-held sprayersto carry out field spraying experiments on the seedlings and young trees of cloves andfound that the application of electrostatic spraying treatment could increase the amount ofpesticides applied per unit area of plants while reducing the labor intensity [12]. After theapplication of electrostatic spray technology in hand-held and knapsack sprayers, it hasalso achieved success in the application of ground agricultural machinery equipment ingreenhouses, orchards, and other operating scenarios [13,14]. A large number of studieson the effects of electrostatic spray equipment have been carried out on droplet size,droplet deposition distribution in each part of crop canopy, nozzle structure, and workingparameters on droplet quality, etc. [15–19].
In 2005, China introduced the electrical parts and nozzles of the aerial electrostaticspray system from the SES company and carried out simulation tests and flight tests [20],marking the beginning of China’s research in the field of aerial electrostatic spraying.Two years later, universities and scientific research institutions in China began to com-prehensively study the aerial electrostatic spray technology based on manned aircraftplatforms in aspects of hardware structure improvement, mechanism of influencing factors,and theoretical simulation.
2.1. Hardware Structure Improvement
Hardware structure improvement research refers to the adjustment or structuralmodification of system parameters such as electrode material, nozzle size, rotor number,and nozzle position of aerial electrostatic spray systems [21].
In 2007, Ru et al. [22] introduced a structured design on a double-nozzle for anaerial electrostatic sprayer and theoretically analyzed the space field induced by thedouble-nozzle and the impact on the size and charging droplets from the space field.
Appl. Sci. 2021, 11, 4071 3 of 15
Zhou et al. [23] improved the design of an aerial electrostatic single-nozzle from the as-pects of an electrostatic electrode, nozzle material, nozzle processing technology, connectionmode of high voltage wire and electrode, rotating screw joints, overflow valve body, andso on, in order to meet the requirements for application and large-scale production.
In terms of the electrode of electrostatic nozzles, Ru et al. [24] modified the originalcylindrical electrode to a cone-shaped electrode according to the features of aerial electro-static spraying. The effect of charging voltage on the charge to mass ratio and depositiondistribution of the new aerial electrostatic system was tested under simulated flight con-ditions. It was found that the electrostatic spray was beneficial to increase the averagedeposition of charged droplets on the lateral, bottom, and back sides of the neutral targetsignificantly. Jin et al. [25] improved the ring electrode of an aerial electrostatic nozzle andconducted an experimental study on the droplet size and the charging effect of dropletsafter the improvement. Results showed that droplet size was influenced by nozzle diam-eter, spraying pressure, and charging voltage, of which the spray pressure indicated thestrongest effect, and charging voltage showed the weakest effect. The charge to mass ratioincreased with the increase in voltage, reaching a maximum of 2.09 mC/kg, and tends tosaturation at 8 kV. The charge to mass ratio decreased with the increase in droplet size,but the change was not rapid.
2.2. Research on Mechanism of Influencing Factors
The settling process of charged droplets is restricted by environmental factors, control-lable factors, and target parameters during spraying operation, thus affecting the operationquality of the electrostatic spraying system. Among these influencing factors, environmen-tal factors include temperature, humidity, wind speed, soil, etc.; controllable parametersinclude charging voltage, spraying flow, spraying pressure, flight altitude, flight speed,etc.; target parameters include crop objects, target morphology, leaf inclination, and insectpest types and habits, and so on [21].
Yang et al. [26] studied the influence of different crosswinds wind speed conditionsand electrostatic voltage on charging characteristics through indoor simulation of naturalwind and constant wind environment, providing a basis for strengthening the anti-driftability of charged spray droplets in the settling process. Chen et al. [27] analyzed thecharacteristics of the electrostatic field induced by the ring electrode with the help of Fluentsoftware (Version 6.3). It was found that the higher charging voltage or smaller electrodespacing (when the distance between the induction electrode ring and the nozzle was lessthan 10 mm) could effectively improve the charge effect and spray quality.
3. Rapid Progress of UAV-Based Electrostatic Spray Technology3.1. Research Background
In 2014, China’s agricultural pesticide application was still dominated by large fixed-wing aircraft, supplemented by rotary-wing UAVs [28]. However, since 2015, with theurbanization process in China, the rural labor force population has become less and less.Meanwhile, with the improvement of living standards, automatic pesticide applicationtools have gradually been accepted by farmers [29]. Nowadays, plant protection UAVshave maintained a booming trend in the field of agricultural plant protection. China hastaken an internationally leading position in terms of technology, quantity, and producttypes of agricultural plant protection UAVs [30].
Figure 1 shows the increasing number of plant protection UAVs and operating areascovered in China from 2014 to 2020. The number of plant protection UAVs has reached110,000 units with an operation area covered of 67 million hm2 in 2020. In this context,Chinese scholars began to put forward the idea of applying aerial electrostatic spraytechnology to UAVs, while there are few reports on this topic worldwide.
Appl. Sci. 2021, 11, 4071 4 of 15
Figure 1. Increasing number of plant protection UAVs and operating area covered in China (2014–2020).
3.2. Beginning of the Research on UAV-Based Electrostatic Spray Technology
In 2015, Ru et al. [31] tried the first combination of electrostatic spray technologyand plant protection UAV with an XY8D unmanned helicopter and carried out field testsin a rice field, as shown in Figure 2. A preliminary experiment on spray pressure, flowrate, and charging voltage were conducted to determine the optimal parameters for fieldtests. Field spraying test results showed that the droplet deposition and coverage rate onthe target crop was effectively improved by the UAV-based electrostatic spraying whencompared with non-electrostatic spraying. The influence of environmental factors andphysical properties of the liquid was not verified in this experiment. However, the firststudy of carrying the electrostatic spray system on a UAV was of great value. Chinesescholars then began to make continuous innovations and breakthroughs in UAV-basedelectrostatic spray technology.
Figure 2. Rice field test with an XY8D UAV-based electrostatic spray system [31].
Appl. Sci. 2021, 11, 4071 5 of 15
3.3. Continuous Optimization of UAV-Based Electrostatic Spray Technology
Chinese researchers have carried out a series of optimization work from the aspectsof nozzle modification, test effect comparison, and mechanism of influencing factors inorder to meet the precise operation requirements of UAV-based electrostatic spray systemin practical application and to improve the droplet deposition on the target crops.
In terms of nozzles modification, Wang et al. [32] improved the conventional nozzlesfor multi-rotor plant protection UAVs. An inductive type electrostatic centrifugal nozzlewas developed by combining agricultural electrostatic spray technology with a centrifugalatomizing nozzle. Besides, the spraying flow stability test and droplet deposition effect ofthe system were studied. When the charging voltage of the nozzle reaches 8 kV, the chargeto mass ratio reaches the maximum value of 0.59 mC/kg.
In terms of comparison with the test results of non-electrostatic spray, Jin et al. [33]from Nanjing Forestry University designed an electrostatic spray system for the AF-118 helicopter. Through the effective spraying width and droplets deposition character-istics, it is found that the effective spraying width of electrostatic spray was smallerthan that of non-electrostatic spray. The total droplet deposition number of electrostaticspray (9–36 drops/cm2) was higher than that of non-electrostatic spray (6–26 drops/cm2).Lian [34] used YG-6 six-rotor UAV to carry an electrostatic spray system. Through anindoor performance test, the optimal operating parameter combination of the system wasdetermined (spraying height is 50 cm, charging voltage is 9 kV, spraying pressure was0.3 MPa). This parameter combination was used to test the electrostatic spray effect ofoutdoor UAVs. The result showed that the average deposition density of droplets sprayedabove the target by electrostatic spray was 16.1 more/cm2, 13.6% higher than that bynon-electrostatic spray. The average sediment density in the middle was 28 more/cm2,which increased by 32.6%. Cai [35] developed an aerial electrostatic spray system basedon an F-50 plant protection UAV. The spray system adopts contact charge, and the exper-imental research is carried out according to the factors of the rotor wind field, such aswind speed, charge to mass ratio, flight height, and flight speed. Through field sprayingexperiments, as shown in Figure 3, it was found that the flight height is an important factoraffecting the deposition amount and the horizontal deposition uniformity but has littleeffect on the vertical deposition uniformity. Zhao et al. [36] proposed a method of chargingthe liquid in two isolated water tanks with positive and negative charges respectivelyby a high-voltage electrostatic generator based on the contact charge mode to solve theproblem of insufficient adsorption rate of droplets on the target back when using an aerialelectrostatic spray. Aerial electrostatic spraying test stand and UAV electrostatic spraysystem were designed, which proved that it was feasible to develop a charge transfer loopin space to improve the adsorption performance of droplets. An electrostatic physicalmodel of aerial electrostatic spray based on charge transfer space loop is shown in Figure 4.Zhang et al. [37] developed a fan-shaped induction electrostatic spray system based on asix-rotor UAV and defined the corresponding operating parameters (spray height 50 cm,spray pressure 0.3 MPa, and charging voltage 9 kV). Compared with non-electrostaticspray, the electrostatic spray had more concentrated droplet deposition and smaller drift.The average droplet deposition density at the top of the electrostatic spray sampling devicewas 16.1 drops/cm2 higher than that of non-electrostatic spray, and the deposition densityin the middle was 28 drops/cm2 higher than that of non-electrostatic spray. Therefore,aerial electrostatic spray could significantly improve droplet deposition and prevent drift.
Appl. Sci. 2021, 11, 4071 6 of 15
Figure 3. UAV paddy field spraying operations [35].
Figure 4. Electrostatics physical model of aerial electrostatic spraying based on charge transfer spaceloop [36].
In terms of the mechanism of influencing factors, Bu [38] designed an electrostaticspray system based on the FR-200 large-load unmanned helicopter with a maximumload of 80 kg. The charge and spray characteristics of the electrostatic spray system werestudied, and the prediction models of the charge to mass ratio and droplet size wereestablished. The characteristics of deposition and drift of electrostatic spray of heavy loadunmanned helicopter were analyzed by field experiments. It was concluded that the chargevoltage, flight height, and crosswind wind speed were the main factors affecting the driftand deposition of an electrostatic spray of FR-200. The results showed that the chargevoltage had the greatest influence, followed by the crosswind wind speed, and the flightheight had the least influence. Based on the research results, the mathematical modelwas established between the center distance of droplet mass and droplet drift rate andthe charged voltage, flight height, and crosswind wind speed. Wu et al. [39] introducedresponse surface analysis (RSM) into the optimization design of spray parameters ofElectrostatic spray system of UAVs. A response surface model with injection pressureand nozzle diameter as design variables and droplet charge to mass ratio as optimizationobjectives was constructed. The performance tests of the new electrostatic spray systemunder different nozzle diameters, spray pressures, and electrostatic voltages were carriedout. It showed that the nozzle met the theoretical requirements of optimal biological particle
Appl. Sci. 2021, 11, 4071 7 of 15
size and the requirements of hydraulic spray control for most crop diseases and insect pests.The validity of the model was also proved. In addition, multivariate analysis of varianceshowed that nozzle diameter, spray pressure, and electrostatic voltage have importanteffects on performance. The influence of the two factors on the deposition density anduniformity is the spray pressure, the nozzle diameter, and the optimal combination ofthe factor levels to obtain the best results. Chinese scholars also carried out research onmultiple operating parameters and their interaction effects on the performance of aerialelectrostatic spray systems. Lu et al. [40] simulated the spray performance of UAV-basedelectrostatic spray systems at different flight heights by measuring the droplet diameterunder different nozzle apertures and system pressures and obtained the optimal flightparameter combination. Zhao et al. [41] studied the spray deposition with three factors:spraying duration, charging voltage, and flight height. The experimental results showedthat the back-front ratio of droplets on the back and front of leaf targets could reach 158.8%under indoor conditions, and the droplet size on the back was smaller than that on thefront. The number of droplets increased with the accumulation of spraying time withoutaffecting the back-front ratio. Higher charging voltage and lower spraying height for theaerial electrostatic spray system can achieve a better deposition effect and higher back-frontratio. Wang et al. [42] established theoretical equations of droplet group charge based onthe water inductive charging theory and then studied the effects of electrode figuration(electrode ring diameter, electrode spacing, spray pressure, and charge voltage) on dropletcharging and spray performance through experiments. Under the action of an electrostaticfield, the droplet size decreased obviously with the increase in charged voltage. The chargeperformance improved with the decrease in electrode ring diameter. Lan et al. [43] studiedthe impact of electrode materials on the deposition characteristics of an aerial electrostaticsystem with different orifice sizes, system pressures, and charging voltages. The resultsshowed that red copper was the best electrode material. Li et al. [44] simulated five factors,such as temperature, humidity, electrode ring diameter, electrostatic voltage, and nozzleflow, using a BP model based on Neuroshell software and studied the effects on the chargeto mass ratio of airborne electrostatic droplets. The final linear regression model indicatedthat the charging voltage and flow rate were the two main influencing factors on the dropletcharge to mass ratio.
3.4. Spray Efficiency Experiments of UAV-Based Electrostatic Spray Technology
Wang et al. [45] designed a bipolar contact oil-powered single rotor aerial electrostaticspray system for plant protection UAVs. The static electricity system was used to spraythe static electricity oil agent and the conventional water-based chemical agent. Besides,the spray droplet deposition distribution and the control effect of wheat aphid and rustwere tested. The result showed that the deposition per unit area was 0.0486 µg/cm2,the standard deviation of deposition amounts was 0.015 µg/cm2, and the coefficientof variation was 30.43%. The distribution uniformity of droplet deposition is obviouslybetter than the other two treatments, and the prevention and control of diseases andinsect pests in the wheat field showed a good control effect. The aphid control effect was87.92% on the seventh day after spraying, which was significantly higher than that of theconventional spray treatment (76.43% with the electrostatic oil agent and 66.47% with theconventional water-based agent). Liu [46] designed a high-voltage electrostatic generatorwith smaller volume and mass based on the optimized structure parameters of the cone-shaped electrode and the determined dosage form of the special electrostatic liquid agentfor aviation. It was more suitable for plant protection UAVs, aimed at the current problemssuch as the poor charging effect of droplets, poor applicability of aerial spraying agents,complex high-voltage electrostatic generator system, and heavy high-voltage electrostaticgenerator. In addition, a set of electrostatic spray systems that can be applied to six-rotorand single-rotor plant protection UAVs was developed, as shown in Figure 5. The systemwas mounted on the plant protection UAV and carried out the experiment in the cotton andrice-growing areas of Changji, Xinjing, Shihezi, Xinjing, and Ledong, Hainan. The results of
Appl. Sci. 2021, 11, 4071 8 of 15
spraying operation of hybrid breeding rice growth regulator and defoliating cotton agent byplant protection UAV showed that when spraying rice growth regulator by plant protectionUAV, the effect of electrostatic spraying could be up to 20% higher than that of non-staticspraying, and the effects of spraying different chemicals were different. When sprayed thecotton defoliant, the coverage rate of droplet increased by 140%, the defoliation efficacyincreased by 12.22%, and the cotton boll opening rate increased by 18.55%.
Figure 5. HY-B-15L single-rotor plant protection unmanned helicopter for cotton defoliant spray (a) and TXA616 plantprotection UAV for rice growth regulator spray (b) [46].
3.5. Summary
To sum up, a large number of field experiments have been conducted in China totest the actual performance of the electrostatic spray system based on UAVs. For theconvenience of comparative analysis, Table 1 summarizes the researches on UAV-basedelectrostatic spray systems. From the perspective of the test scheme, the system pressure,flight height and speed, charging voltage, rotor wind speed, and their effects on eachother were studied. Chinese scholars have also completed a lot of work using theoreticalsimulations of the spray system, the improvement of the electrostatic nozzle, and theoptimization of the aerial chemicals. In terms of the mode of charge, most studies adoptthe induction mode of charge with the highest safety. The inductive charging voltage isusually between 2 kV and 15 kV, and the electrode making and insulation methods are easyto be realized, which is the most developed method of charging droplets at present [47].In addition, contact charging has also been tried in China, which has also achieved goodresults [34–36,41,45]. However, it needs to keep the absolute insulation of the spray systemin the charging process, which puts forward high requirements for the design methodand safety.
Figure 6 illustrates the key achievements of the aerial electrostatic spray technologyand the research groups over a timeline. Since 2015, China’s UAV-based aerial electrostaticspray technology has developed rapidly. In general, although the UAV-based electrostaticspray technology is a new technology, it is gradually being improved. Compared with theelectrostatic spray system on manned aircraft, plant protection UAVs have more develop-ment prospects in China at present. With the operation area covered exceeding 67 millionhm2, the plant protection UAV has developed from an early experimental product into acommon agricultural production machine in China [48]. It is of great practical significanceto study the fusion and influence mechanism of UAV and aerial electrostatic spray systems,which contributes to the field of agricultural aviation plant protection in the future.
Appl. Sci. 2021, 11, 4071 9 of 15
Table 1. Study on UAV-based Electrostatic Spray System.
Test Device Charged Mode Test Target Test Method Evaluating Indicator Test Environment Researchers and Affiliation
XY8D UAV Induction Rice Different flight height Droplet depositioncharacteristics Field test Ru, et al., Nanjing Forestry
University
AF-811 unmannedhelicopter Induction Copy paper
Comparison ofelectrostatic spray andnon-electrostatic spray
Effective sprayamplitude and droplet
depositioncharacteristics
Outdoor Jin, et al., Nanjing ForestryUniversity
YG20-6 typesix-rotor UAV Induction Water sensitive paper
Comparison ofelectrostatic spray andnon-electrostatic spray
Measurement of effectivespray amplitude, dropletpenetration, and droplet
deposition
Outdoor Lian, Heilongjiang BayiAgricultural University
F-50 plant-basedunmanned helicopter Contact Sample tape, rice
Comparison of differentcharge modes
(non-electrostatic,induction electrostatic,contact electrostatic)
Droplet depositionmeasurement and
coefficient of variation ofdeposition amount
Field test Cai, Jiangsu University
HY-B-15L single rotorplant protection
unmanned helicopter,TXA micro
six-rotor UAV
InductionCrossbreeding rice,Cotton picked by
machine
Comparison of differentwater agents, number of
rotors, and crops
Droplet depositioncharacteristics and
pharmacodynamicsField test Liu, South China Agricultural
University
Integrated spraytest stand Induction Water sensitive paper
Comparison ofelectrostatic spray andnon-electrostatic spray
Density of dropletdeposition Indoor Zhang et al., Heilongjiang
Bayi Agricultural University
3WQF120-12oil-operated single-rotor
plant protection UAVContact wheat
Comparison betweenelectrostatic oil agent
and conventionalwater-based agent,
electrostatic spray andconventionalspray system
Distribution uniformityof droplet deposition
and its control effect onwheat aphid and rust
Field test Wang et al., China AgricultureUniversity
FR-200 type large-loadunmanned helicopter Induction Water sensitive paper
Charged voltage, flightspeed, flight height, andcrosswind wind speed
Droplet depositioncharacteristics anddroplet drift rate
Field test Bu, Jiangsu University
Appl. Sci. 2021, 11, 4071 10 of 15
Table 1. Cont.
Test Device Charged Mode Test Target Test Method Evaluating Indicator Test Environment Researchers and Affiliation
response surfacemethodology (RSM) Induction -
Nozzle diameter, nozzlepressure, and
electrostatic voltage
Droplet depositioncharacteristics and
charge to mass ratioSimulation Wu et al., Guizhou University
Gaoke M45 plantprotection UAV Contact Mock target
Comparison ofelectrostatic spray andnon-electrostatic spray
Droplet depositioncharacteristics Outdoor Zhao et al., Shandong
University of Technology
W730S four-rotorsemi-automatic plant
protection UAVInduction Effusion cylinder -
Droplet depositionmeasurement and spray
flow stabilityIndoor Wang et al., Hangzhou Dianzi
University
An indoor spray barelectrostatic spray
system test platformInduction Water sensitive paper
Nozzle diameter, nozzlepressure, and
spraying height
Droplet depositioncharacteristics Indoor Lu et al., Guizhou University
Aerial electrostaticspraying system
suspension test benchContact Art Paper
Spraying height,spray time,
and charging voltage
Droplet depositioncharacteristics and
back-front ratioIndoor Zhao et al., Shandong
University of Technology
Charge aerosol spraytest bench Induction White plastic board
Charging voltage, spraypressure, electrode ring
diameter,and electrode spacing
Spray charge-to-massratio and droplet size Indoor Wang et al., Jiangsu University
Fixed spray simulator Induction Dracaena
Electrode material,charging voltage,
spray pressure, andnozzle diameter
Droplet deposition Indoor Lan et al., South ChinaAgricultural University
Neuroshell Induction -
Temperature, humidity,electrode ring diameter,charging voltage, and
nozzle flow
Spray charge-to-mass ratio Simulation Li et al., Heilongjiang Bayi
Agricultural University
Appl. Sci. 2021, 11, 4071 11 of 15
Figure 6. Key milestones and research groups working on aerial electrostatic spray technology.
4. Analysis and Prospects
The exploration of UAV-based electrostatic spray technology expands a new researchperspective for the research of agricultural aviation electrostatic spray technology. The con-tinuous improvement of the technology brings new opportunities for the application ofaerial electrostatic spray systems on commercial plant protection UAVs in China. However,according to the current research progress, still, the following key technical elements needfurther exploration.
4.1. Measurement Technology of Charged Droplets
Droplet charge to mass ratio is a term associated with aerial electrostatic spray technol-ogy, and the measurement results of charge to mass ratio provide a reference for the evalua-tion of electrostatic spray system performance. In the absence of interference (e.g., indoors),the droplet charge to mass ratio is positively correlated with the deposition effect [49].However, when working in an outdoor environment, there is usually a big differencebetween the droplet charge to mass ratios at the nozzle end and the terminal target, whichrequires the terminal measuring device to evaluate the charge amount. However, due tothe absence of electrical grounding, the device that collects charged droplets often results inan inconsistent result with the actual effect of settling on the target [50]. For the electrostaticspray technology, the electric field intensity varies with the plant form, liquid conductivity,and environmental factors. For example, the electric field near the tip or terminal part ofthe leaf is the strongest [51]. Law [52] reported that gaseous discharges between sharp leaftips and incoming charged spray clouds had been shown to limit deposition.
In many studies, aerial electrostatic spray had a better deposition effect comparedwith conventional electrostatic spray and non-electrostatic spray, but there are not a fewreports that the effect of electrostatic spray is not satisfactory [1,49,53]. However, it remainsto be further confirmed whether the difference in operation effect is caused by a sharpreduction in the charge to mass ratio of charge droplets, faulty experimental design scheme,or environmental influence. In recent years, more detection methods such as laser particlesize analyzer have been continuously applied to the measurement of droplets effect in
Appl. Sci. 2021, 11, 4071 12 of 15
China [54,55]. However, there are many problems such as high manufacturing cost, largemeasurement error, and complex measurement process. Therefore, it is necessary to developa droplet characteristic detection technology suitable for aerial electrostatic spray systemsto monitor the real state of charged droplets.
4.2. Impact of UAV Rotor Wind Field on Charged Droplets
The wind field on a manned fixed-wing aircraft causes charged droplets to settle in thedirection of flight towards the area over which the aircraft passes. However, the wind fieldof a multi-rotor plant protection UAV is chaotic and changeable. It is not a regular wind fieldin a single direction. Moreover, due to the dual interference of ambient wind and rotor windfield, the charged droplet deposition process is more complicated in the actual operationscenario. Existing studies have basically ignored the working state of bipolar electrostaticspray system under the action of multiple rotor wind fields, the attracting process ofpositive and negative charged droplets, and the influence of droplets on the humidity ofinductive charging electrodes. However, the impact is huge in that the characteristic ofcharged droplet property undoubtedly loses if the attraction of the charged droplets withpositive and negative polarity is affected. In addition, wet electrodes will also make theelectrically charged performance worse. Therefore, the influence of UAV rotor wind fieldson charged droplets should be paid more attention in future research.
PIV (Particle Image Velocimetry) and other techniques can be used to simulate thedroplet settling state under the influence of the rotor wind field, natural environmentalwind, multiple gradient crosswinds, and other factors, so as to establish a theoretical systemfor reducing the influence of the rotor UAV downwash wind field and even utilizing thewind field action.
4.3. Comparative Study of Various Charging Methods
The induction charging mode was determined from the early stage of aerial electro-static spray technology research in the United States, and it has been adopted in commercialelectrostatic spray systems. Induction charging has the advantages of low charging voltageand low application threshold. It is a safe and effective method for charging droplets.Most Chinese scholars have also applied the induction charging method in their research.However, in addition to induction, there are also contact and corona ways to chargedroplets. Corona charging voltage is very high, up to 30 kV. It can be used for conductiveand non-conductive liquids with low insulation requirements. The contact charging voltageis required to be between corona type and induction type, but the insulation requirementsare higher. Although the corona type with high charging voltage and the contact typewith high insulation requirements still need a lot of basic research to clarify the chargingmechanism and eliminate application risks, the disadvantages of the induction chargingmode with poor charging effect cannot be ignored. Previous studies have shown thatthe contact charging method is able to generate a larger target current when comparedwith the induction charging method [47]. In recent years, there have been reports oncontact charging methods in the research of UAV-based electrostatic spray technology withgratifying test results. In the future, it is necessary to carry out comparative studies ofvarious charging methods in order to evaluate the operation effect of aerial electrostaticspray technology under different charging methods.
4.4. Accumulation of Aerial Electrostatic Spray Technology Research Based on Agricultural UAVs
During the early stage of China’s agricultural aviation electrostatic spray technologyresearch, the design and test were carried out on manned fixed-wing aircraft and heli-copter platforms. In recent years, plant protection UAVs have provided Chinese growerswith a lower barrier to entry and a higher level of applicability. Chinese scholars havecarried on beneficial exploration for UAV-based electrostatic spray technology according todifferent UAV models, field crops, operating parameters, and electrostatic nozzle parame-ters, but the main research emphasis is still on the contrast test of electrostatic spray and
Appl. Sci. 2021, 11, 4071 13 of 15
non-electrostatic spray to verify the operation effect of charged droplets. The research accu-mulation of airborne electrostatic spray technology based on agricultural UAV platforms isstill less. It is not mature at the application level because the morphological characteristicsof various crops and the farmland environment need more experimental data support.For example, (1) Relevant studies on the system construction, composition, and weightcontrol of the electrostatic spray system lack continuity; (2) whether the operation withaerial electrostatic spray system is affected by surrounding facilities such as high-voltagelines, or whether it is incompatible with the flight control system of high-precision andfully autonomous plant protection UAV, is still unknown; (3) at present, the practicalapplication of aerial electrostatic spray system in the field is limited to spraying water andwater-based pesticides, and there is a lack of in-depth discussion on the research of thespecial electrostatic liquid pesticides and the electrical conductivity of pesticides for aerialspraying application; (4) working parameters of the spray system, such as charging voltage,system pressure and spraying speed, selection of flight speed and altitude, characteristics ofaviation agents, and environmental factors such as temperature, humidity, and wind speed,all affect the settling process of charged droplets. Therefore, it is necessary to observe thedroplet characteristics by studying their interaction effects; (5) the development trend ofmost commercial agricultural UAVs in China is integrating pesticide spraying, seed sow-ing, and fertilizer spreading together with fully autonomous and high-precision operation.Therefore, it is necessary to consider whether the integration of aerial electrostatic spraysystem, UAV working systems, and flight control system will cause mutual interference.
5. Conclusions
The development of aerial electrostatic spray technology in China, especially UAV-based aerial electrostatic spray technology, was analyzed in this review from nozzle mod-ification, technical feasibility tests, mechanisms influencing each factor, and field sprayefficiency tests. According to the literature retrieved, the research of China’s aerial electro-static spray technology in the past five years has focused on the innovative exploration ofUAVs as a platform. Combined with the current development of agricultural plant protec-tion and industrial application practice, UAV-based aerial electrostatic spray technologyhas wider developmental potential in China. In the future, the development plans shouldbe made around the basic research, field test, commercialization, demonstration, and ser-vice guidance. In addition, researchers are recommended to pay attention to the integrateddesign of UAV and aerial electrostatic spray systems and to formulate the applicationstandard of aerial electrostatic spray technology in China.
Author Contributions: Conceptualization, Y.Z., Y.L., and X.H.; methodology, X.H., Y.Z., and W.Z.;software, X.L. and W.Z.; validation, X.H. and W.Z.; formal analysis, X.H. and L.W.; investigation, Y.Z.,X.H., and L.W.; resources, X.H.; data curation, X.H.; writing—original draft preparation, X.H., K.Y.,and Y.Z.; writing—review and editing, Y.Z. and K.Y.; visualization, X.L.; supervision, W.Z. and J.D.;project administration, Y.Z., Y.L., and J.D.; funding acquisition, Y.Z. All authors have read and agreedto the published version of the manuscript.
Funding: This research was funded by Key Field Research and Development Plan of GuangdongProvince, China, grant number 2019B020221001, Science and Technology Plan Project of GuangdongProvince, China, grant number 2018A050506073, Guangdong Modern Agricultural Industry GenericKey Technology Research and Development Innovation Team Project, grant number 2020KJ133,National Key Research and Development Program, grant number 2018YFD0200304, and the 111Project, grant number D18019.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
Appl. Sci. 2021, 11, 4071 14 of 15
References1. Zhang, Y.L.; Lan, Y.B.; Fritz, B.K.; Xue, X.Y. Development of aerial electrostatic spraying systems in the United States and
applications in China. Trans. Chin. Soc. Agric. Eng. 2016, 32, 1–7, (In Chinese with English abstract). [CrossRef]2. Martin, D.E.; Latheef, M.A. Efficacy of electrostatically charged glyphosate on ryegrass. J. Electrost. 2017, 90, 45–53. [CrossRef]3. Tavares, R.M.; Cunha, J.P.; Alves, T.C.; Bueno, M.R.; Silva, S.M.; Zandonadi, C.H. Electrostatic spraying in the chemical control of
Triozoida limbata (Enderlein) (Hemiptera: Triozidae) in guava trees (Psidium guajava L.). Pest Manag. Sci. 2016, 73, 1148–1153.[CrossRef] [PubMed]
4. Martin, D.E.; Latheef, M.A.; López, J.D. Electrostatically charged aerial application improved spinosad deposition on early seasoncotton. J. Electrost. 2019, 97, 121–125. [CrossRef]
5. Carlton, J.B.; Isler, D.A. Development of a device to charge aerial sprays electrostatically. Agric. Aviat. 1966, 8, 44–51.6. Carlton, J.B. Technique to Reduce Chemical Usage and Concomitant Drift from Aerial Sprays. United States Department of.
Agriculture Patents No. 5975425, 2 November 1999.7. Spectrum Electrostatic Sprayers, Inc. Available online: http://spectrumsprayer.com/company.html (accessed on 11 May 2020).8. Inculet, I.I.; Fischer, J.K. Electrostatic aerial spraying. IEEE Trans. Ind. Appl. 1989, 25, 558–562. [CrossRef]9. Da Cunha, J.P.A.R.; Barizon, R.R.M.; Ferracini, V.L. Assalin, Spray drift and caterpillar and stink bug control from aerial
applications with electrostatic charge and atomizer on soybean crop. Eng. Agrícola 2017, 37, 1163–1170. [CrossRef]10. Martin, D.E.; Latheef, M.A.; Lopez, J.D.; Duke, S.E. Aerial Application Methods for Control of Weed Species in Fallow Farmlands
in Texas. Agronomy 2020, 10, 1764. [CrossRef]11. Anon. Introduce a new technology of pesticide application-electrostatic spray. For. Sci. Technol. 1974, 1, 18.12. Anon. Preliminary test of pesticide electrostatic spray. For. Sci. Technol. 1978, 2, 16–17.13. He, X.K.; Yan, K.R.; Chu, J.Y.; Wang, J.; Zeng, A.J.; Liu, Y.J. Design and testing of the automatic target detecting, electrostatic,
air assisted, orchard sprayer. Nongye Gongcheng Xuebao Trans. Chin. Soc. Agric. Eng. 2003, 19, 78–80, (In Chinese with Englishabstract).
14. Yu, Y.C.; Wang, B.H.; Shi, J.Z.; Li, X.F. Design and experimental study of combined-charging hydraulic electrostatic spraying box.Trans. Chin. Soc. Agric. Eng. 2005, 21, 85–88, (In Chinese with English abstract).
15. Zhang, J.; Zheng, J.Q. Experiment on the variations of droplet diameter distribution and local flow rate over radial direction forthe electrostatic spray process. Trans. Chin. Soc. Agric. Eng. 2009, 25, 104–109, (In Chinese with English abstract). [CrossRef]
16. Jia, W.D.; Hu, H.C.; Chen, L.; Chen, Z.G.; Wei, X.H. Performance experiment on spray atomization and droplets deposition ofwind-curtain electrostatic boom spray. Trans. Chin. Soc. Agric. Eng. 2015, 31, 53–59, (In Chinese with English abstract). [CrossRef]
17. Zhou, L.F.; Zhang, L.; Xue, X.Y.; Qin, W.C.; Sun, Z.; Zhou, Q.Q.; Chen, C. Design and test of double air channel auxiliaryelectrostatic sprayer. Jiangsu Agric. Sci. 2017, 45, 192–196, (In Chinese with English abstract). [CrossRef]
18. Ma, X.; Guo, L.J.; Wen, Z.C.; Wei, Y.H.; Xiao, R.H.; Zeng, H. Atomization characteristics of multi-nozzle electrostatic spray andfield experiment. Trans. Chin. Soc. Agric. Eng. 2020, 36, 73–82, (In Chinese with English abstract). [CrossRef]
19. Dai, Q.F.; Hong, T.S.; Song, S.R.; Li, Z.; Chen, J.Z. Influence of pressure and pore diameter on droplet parameters of hollow conenozzle in pipeline spray. Trans. Chin. Soc. Agric. Eng. 2016, 32, 97–103, (In Chinese with English abstract). [CrossRef]
20. Luan, H.; Zhang, Q.; Wang, W.X. Installation and modification of Z03K000B electrostatic spray system and flight tests. Xin-jiangReclam. Technol. 2006, 5, 46–47, (In Chinese with English abstract).
21. Zhang, Y.L.; Huang, X.R.; Wang, L.L.; Deng, J.Z.; Zeng, W.; Lan, Y.B.; Muhammad, N.T. Progress in foreign agricultural aviationelectrostatic spray technologies and references for China. Trans. Chin. Soc. Agric. Eng. 2021, 37, 50–59, (In Chinese with Englishabstract). [CrossRef]
22. Ru, Y.; Zheng, J.Q.; Zhou, H.P.; Shu, C.R. Design and experiment of double nozzle of aerial electrostatic sprayer. Trans. Chin. Soc.Agric. Mach. 2007, 38, 58–61.
23. Zhou, H.P.; Ru, Y.; Shu, C.R.; Jia, Z.C. Improvement and experiment of aerial electrostatic spray device. Trans. Chin. Soc.Agric. Eng. 2012, 28, 7–12, (In Chinese with English abstract). [CrossRef]
24. Ru, Y.; Jin, L.; Zhou, H.P.; Shu, C.R. Effect of high-voltage electrostatic field on droplet charging based on cone-shaped electrode.High Volt. Eng. 2014, 40, 2721–2727, (In Chinese with English abstract). [CrossRef]
25. Jin, L.; Ru, Y.; Sun, M.L.; Jia, Z.C.; Wang, B.X. Performance experiments of aerial electrostatic nozzle with cone shaped electrode.J. Nanjing For. Univ. (Nat. Sci. Ed.) 2015, 39, 155–160, (In Chinese with English abstract). [CrossRef]
26. Yang, Z.; Niu, M.M.; Li, J.; Xu, X.; Sun, Z.Q.; Xue, K.P. Influence of lateral wind and electrostatic voltage on spray drift ofelectrostatic sprayer. Trans. Chin. Soc. Agric. Eng. 2015, 31, 39–45, (In Chinese with English abstract). [CrossRef]
27. Chen, H.L.; Zhao, Y.C. Electrostatic induction field and charging property of droplet in electrostatic spraying process.High Volt. Eng. 2010, 36, 2519–2524, (In Chinese with English abstract). [CrossRef]
28. Zhang, D.Y.; Lan, Y.B.; Chen, L.P.; Wang, X.; Liang, D. Current status and future trends of agricultural aerial spraying technologyin China. Trans. Chin. Soc. Agric. Mach. 2014, 45, 53–59, (In Chinese with English abstract). [CrossRef]
29. Tian, Z.W.; Xue, X.Y.; Li, L.; Cui, L.F.; Wang, G.; Li, Z.J. Research status and prospects of spraying techology of plant-protectionunmanned aerial vehicle. J. Chin. Agric. Mech. 2019, 40, 37–45, (In Chinese with English abstract). [CrossRef]
30. Li, J.Y.; Lan, Y.B.; Shi, Y.Y. Research progress on airflow characteristics and field pesticide application system of rotary-wing UAV.Trans. Chin. Soc. Agric. Eng. 2018, 34, 104–118, (In Chinese with English abstract). [CrossRef]
Appl. Sci. 2021, 11, 4071 15 of 15
31. Ru, Y.; Jin, L.; Jia, Z.C.; Bao, R.; Qian, X.D. Design and experiment on electrostatic spraying system for unmanned aerial ve-hicle.Trans. Chin. Soc. Agric. Eng. 2015, 32, 42–47, (In Chinese with English abstract). [CrossRef]
32. Wang, Y.T.; Wu, K.H. Research on an electrostatic spraying system for multi-rotor plant protection UAV. Jiangsu Agric. Sci. 2020,48, 225–230, (In Chinese with English abstract). [CrossRef]
33. Jin, L.; Ru, Y. Research on UAV-based aerial electrostatic spraying system. J. Agric. Mech. Res. 2016, 38, 227–230, (in Chinese withEnglish abstract). [CrossRef]
34. Lian, Q. Experimental Research on Electrostatic Spray System of Multi-Rotor UAV. Master’s Thesis, Heilongjiang Bayi AgriculturalUniversity, Daqing, China, 2016.
35. Cai, Y.L. Design and Test of Contact Electrostatic Spray System for Low-Altitude UAV. Master’s Thesis, Jiangsu University,Zhenjiang, China, 2017.
36. Zhao, D.N.; Lan, Y.B.; Shen, W.G.; Wang, S.Z.; Abhishek, D. Development of a charge transfer space loop to improve ad-sorptionperformance in aerial electrostatic spray. Int. J. Agric. Biol. Eng. 2020, 13, 50–55. [CrossRef]
37. Yanliang, Z.; Qi, L.; Wei, Z. Design and test of a six-rotor unmanned aerial vehicle (UAV) electrostatic spraying system for cropprotection. Int. J. Agric. Biol. Eng. 2017, 10, 68–76. [CrossRef]
38. Bu, J.Z. Experimental Study on Deposition and Drift Characteristics of Electrostatic Spray for Large-Load Unmanned Helicopter.Master’s Thesis, Jiangsu University, Zhenjiang, China, 2019.
39. Wu, Y.; Lu, J.; Wang, Y. Research on Multiresponse Robustness Optimization for Unmanned Aerial Vehicle Electrostatic SpraySystem. Fundam. Mach. Theory Mech. 2020, 77, 719–728. [CrossRef]
40. Lu, J.J.; Chen, J.D.; Wu, Y.D.; Wang, B. Agricultural aviation research aeronautical electrostatic spray system performance. J. Agric.Mech. Res. 2019, 41, 174–179, (In Chinese with English abstract). [CrossRef]
41. Zhao, D.N.; Lan, Y.B.; Shen, W.G. Building of aerial electrostatic spraying system and exploration on the influencing factors ofdroplet deposition effect. J. Agric. Mech. Res. 2021, 43, 204–207, (In Chinese with English abstract). [CrossRef]
42. Wang, J.F.; Gu, W.K.; Wang, X.Y.; Chu, J.J. Experimental study on charging performance and spray characteristics of inductioncharged spray. High Volt. Eng. 2015, 41, 504–509, (In Chinese with English abstract). [CrossRef]
43. Lan, Y.B.; Zhang, H.Y.; Wen, S.; Li, S.H. Analysis and experiment on atomization characteristics and spray deposition ofelectrostatic nozzle. Trans. Chin. Soc. Agric. Mach. 2018, 49, 130–139, (In Chinese with English abstract). [CrossRef]
44. Li, Y.F.; Hu, J.; Li, Q.D.; Zhao, M.M. Study on influencing factors of airborne electrostatic fog trapping. Trans. Chin. Soc.Agric. Mach. 2019, 41, 171–176, (In Chinese with English abstract). [CrossRef]
45. Wang, S.L.; He, X.K.; Song, J.L.; Zhong, C.S.; Wang, Z.C.; Qi, P.; Ling, Y. Charging and spraying performance test of bipolarcontact electrostatic spraying system for unmanned aerial vehicle. Trans. Chin. Soc. Agric. Eng. 2018, 34, 82–89, (In Chinese withEnglish abstract). [CrossRef]
46. Liu, W.L. Design and Experiment of Electrostatic Spraying System for Plant Protection Unmanned Aerial Vehicle. Master’s Thesis,South China Agricultural University, Guangzhou, China, 2017.
47. Zhao, S.; Castle, G.P.; Adamiak, K. Comparison of conduction and induction charging in liquid spraying. J. Electrost. 2005, 63,871–876. [CrossRef]
48. Agropages. China’s Crop Protection UAV Development Report 2020. Available online: http://news.agropages.com/News/NewsDetail---38284.htm (accessed on 17 March 2021).
49. Kirk, I.W.; Hoffmann, W.C.; Carlton, J.B. Aerial electrostatic spray system performance. Trans. ASAE 2001, 44, 1089–1092.[CrossRef]
50. Martin, D.E.; Latheef, M.A. Aerial electrostatic spray deposition and canopy penetration in cotton. J. Electrost. 2017, 90, 38–44.[CrossRef]
51. Dai, Y.; Law, S. Modeling the transient electric field produced by a charged pollen cloud entering a flower. In Proceedings of theIAS ’95. Conference Record of the 1995 IEEE Industry Applications Conference Thirtieth IAS Annual Meeting, Orlando, FL, USA,8–12 October 1995; Institute of Electrical and Electronics Engineers (IEEE): New York, NY, USA, 2002; Volume 2, pp. 1395–1402.
52. Law, S.E. Electrostatic Pesticide Spraying: Concepts and Practice. IEEE Trans. Ind. Appl. 1983, 2, 160–168. [CrossRef]53. Martin, D.E.; Lopez, J.D.; Hoffmann, W.C.; Fritz, B.K.; Lan, Y. Field Evaluation of Spinosad Aerial Applications for Thrips Control
on Cotton1. Southwest. Èntomol. 2007, 32, 221–228. [CrossRef]54. Zhang, R.R.; Chen, L.P.; Lan, Y.B.; Xu, G.; Kan, J.; Zhang, D.Y. Development of a deposit sensing system for aerial spraying
application. Trans. Chin. Soc. Agric. Mach. 2014, 45, 123–127, (In Chinese with English abstract). [CrossRef]55. He, Y.; Wu, J.J.; Fang, H.; Zheng, Q.S.; Xiao, S.P.; Cen, H.Y. Research on deposition effect of droplets based on plant protection
unmanned aerial vehicle: A review. J. Zhejiang Univ. (Agric. Life Sci.) 2018, 44, 392–398, (In Chinese with English abstract).[CrossRef]